A technique for forming transistors using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistors are applied to a wide range of electronic devices such as integrated circuits (ICs) or image display devices (display devices). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 1).